Dear Rabbi Simon,
I read in the Kesher Connections weekly newsletter that Shabbat Parashat Shofetim (21-22 August 2020) ends at 9.03pm. But there are at least two other times which some people follow as authoritative. There is a slightly earlier end time of 9.00pm, quoted in the Jewish Chronicle and the Jewish Weekly newspapers and used by the United Synagogue. Then there is the Kesher time of 9.03pm, which corresponds to nightfall. Then there is the Rabbenu Tam time of 9.22pm, which some Chassidim use. I am not aware of other halachic times for the end of Shabbat, but there may be others too. Why do most kehillas in Golders Green, including Kesher, use the time for “nightfall”, rather than the slightly earlier time for the end of Shabbat? What is the halachic basis for the earlier time?
TY for your sophisticated and practical question. Allow me to answer in brief:
The topic of halakhic times is complex and, perhaps unsurprisingly, subject to differing views. Part of the discrepancy lies in how much time (additional minutes) to allow for tosefet Shabbat. (Tosefet [adding to] Shabbat refers to the mitzvah to extend Shabbat for an additional time interval after nightfall itself.) The times we use at Kesher (which are the Adass=Union times) allow 9 minutes. LBD (=United Synagogue) allows approximately 6 minutes. This differential can also vary, summer to winter. In terms of degrees below the horizon (ie how far the sun has descended after sunset), for Rabbinic purposes (eg end of fast days), we use 7.5 degrees, for Torah purposes (eg reciting the night-time shema) 8.0 degrees, for tosefet Shabbat and YT, 8.5 degrees. The time often known as Rabbenu Tam is calculated on a fixed basis of 72 minutes after sunset. (It is interesting to note that while for most of the year, as in the example you cite, the time of Rabbenu Tam [=sunset+72 minutes] is later than the other times, at the height of the summer this is not case. Rather, the Kesher time is 22:37 and Rabbenu Tam is 22:34.)
You may certainly rely on the LBD (=Jewish Chronicle) times if you would like to do so. We prefer the slightly later Shabbat times, reflecting a devotion and love for the sanctity of the day.
I hope this sheds some light on the three stars.
Rabbi Rashi Simon